The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, a vehicle onboard camera, a network camera, a video conferencing camera, a fiberscope, and an encapsulated endoscope.
In these years, there have been available some vehicles equipped with a plurality of cameras for a purpose of enhancing convenience and security. For example, in case of a vehicle equipped with a backup camera to take an image behind the vehicle, since a driver can see the rear view of the vehicle on a monitor upon backing up the vehicle, the driver can safely move the vehicle backward without hitting an object even if any, although such an object is not visible from the driver due to shadow of the vehicle. Such a camera mounted on a vehicle, i.e., a so-called onboard camera, is expected to be continuously on demand.
The onboard cameras are often accommodated in a relatively small space such as in a backdoor, a front grill, a side mirror, and inside of the vehicle. For this reason, an imaging lens to be mounted in the onboard camera is required to have a compact size. Further, the onboard camera is required to be compatible with a high resolution resulting from a high-pixel density imaging element, and to have a wide angle to be compatible with a wide imaging range. However, it is difficult to attain a small size and compatibility with the high resolution while satisfactorily correcting aberrations, and further attain a wide imaging angle. For example, when a size of an imaging lens is reduced, refractive power of each lens tends to become stronger. Accordingly, it is difficult to satisfactorily correct aberrations. Therefore, upon actually designing the imaging lens, it is important to satisfy those demands in a balanced manner.
As a wide-angle imaging lens that has a wide imaging angle, for example, there is known an imaging lens described in Patent Reference. The imaging lens includes a first lens that has a shape of a meniscus lens directing a convex surface thereof to an object side and is negative; a second lens that has a shape of a meniscus lens directing a concave surface thereof to the object side and is positive; a third lens that is positive; and a fourth lens that is positive, arranged in the order from the object side.
Patent Reference: Japanese Patent Application Publication No. 2011-145665
According to the imaging lens disclosed in Patent Reference, the second lens is made of a material having Abbe's number between 23 and 40, and the third lens is made of a material having Abbe's number between 50 and 85. Furthermore, according to the imaging lens, a ratio (f/D) of a focal length f of the whole lens system and a distance D from an incident surface on the object side to an image-forming surface is restrained within certain ranges. Accordingly, it is possible to obtain a wide angle of view and a small size, and also satisfactorily correct a chromatic aberration.
According to the imaging lens disclosed in Patent Reference, although the number of lenses that compose the imaging lens is as few as four, an imaging angle of view is wide and it is also possible to relatively satisfactorily correct aberrations. However, demands for such a wide-angle imaging lens have become diversified each year, and especially in these years, there are strong demands for being capable of manufacturing the imaging lens inexpensively, i.e., an imaging lens that is easy to assemble with high productivity, as well as the demands to be compatible to high-resolution imaging elements and to have a small size.
In case of the conventional wide-angle imaging lens including the imaging lens disclosed in Patent Reference, the first lens has very strong negative refractive power relative to other lenses in order to attain a wide angle of view. For this reason, a curvature radius of an image plane-side surface of the first lens is small, and thereby a so-called semispherical ratio is close to 1.0 (semispherical shape), which results in poor workability of the lens.
Further, the image plane-side surface of the first lens is frequently coated with an antireflection coating or the like, and there is a serious issue of insufficient coating a periphery of the lens surface in case of a lens having the semispherical ratio near 1.0 described above. Furthermore, in case of the imaging lens, in which the first lens has strong refractive power and has the semispherical ratio near 1.0, the sensitivity to deterioration of image-forming performance due to decentering (eccentricity), tilting, etc. occurred upon manufacturing of the imaging lens, i.e., production error sensitivity, is high, and there is a limit by itself for reduction of the manufacturing cost.
Here, those issues are not unique to an imaging lens for mounting on an onboard camera, but are common in imaging lenses for mounting in relatively small cameras, such as cellular phones, digital still cameras, portable information terminals, security cameras, network cameras, video conferencing cameras, fiberscopes, and encapsulated endoscopes.
In view of the above-described problems of conventional techniques, there is provided an invention, an object of which is to provide an imaging lens that has a wide imaging angle of view despite of a small size thereof and can suitably reduce the manufacturing cost.
Further objects and advantages of the present invention will be apparent from the following description of the present invention.